Abstract: A T-carrier fiber optic modem for a telephone central office which allows simple conversion from electrical to optical transmission. An unused slot in a T-carrier span terminating shelf is provided with one optical transmitter/receiver pair for each of the T1 cards in the same shelf. All of the optical fibers are contained in ribbon cables which are connected to this "new" card, with a minimum number of electrical connections being made between the new card and the T1 cards. A minor change in each of the T1 cards "fools" it into thinking that the necessary DC supervisory potential appears on the two receive pins previously connected to a copper twisted-pair cable.